In recent years, tendency of a shorter wavelength of laser bean as a light source which has been used to record and/or reproduce information for optical discs in the optical pickup apparatus, has become a main stream. For example, various blue-violet laser light sources have been made practical. Information of 15 to 20 GB can be recorded on the optical disc having a diameter of 12 cm by using these blue-violet light sources and an objective lens having NA (Numerical aperture) which is the same as a DVD (Digital Versatile Disc). When NA is increased to 0.85, information of about 25 GB can be recorded onto the optical disc having a diameter of 12 cm. In this specification, the optical disc and an optical-magnetic disc using a blue-violet laser light source are called “a high density optical disc”.
Hereupon, the high density optical disc using the objective lens with NA of 0.85, has larger coma caused due to the skew of the optical disc. Therefore some high density optical discs using the objective optical element with NA of 0.85, are designed so that the protective layer has thinner thickness (which is 0.1 mm, while that of DVD is 0.6 mm) than that of DVD to reduce the comma due to the skew. On the other hand, it is sometimes considered that a product, such as an optical disc player/recorder, which is capable of only recording/reproducing information for the above high-density optical disc is worthless. Taking account of a fact that, at present, DVDs and CDs (Compact Disc), onto which various kinds of information have been recorded, are on the market, the value of the product as a high-density optical disc player/recorder is increased by, for example, enabling to appropriately record/reproduce information additionally for DVDs and CDs, which a user possesses. From these backgrounds, the optical pickup apparatus installed in the high-density optical disc player/recorder is required to be capable of appropriately recording/reproducing information not only for a high-density optical disc but also a DVD and a CD.
It can be considered, as a method by which the information can be adequately recorded/reproduced while the compatibility is maintained also to anyone of the high density optical disc and DVD and further to CD, a method to selectively switch the optical system for the high density optical disc and the optical system for DVD and CD corresponding to the recording density of the optical disc to which the information is recorded/reproduced. However, it requires a plurality of optical systems, which is disadvantageous for the size-reduction and which increases the cost.
Accordingly, in order to simplify the structure of the optical pickup apparatus and to intend the reduction of cost, it is preferable to make an optical system for the high density optical disc an optical system for DVD and CD into a common optical system, and to reduce the number of optical parts contributing the optical pickup apparatus as much as possible, even when the optical pickup apparatus has compatibility. Then, providing the common objective optical element which is arranged with facing an optical disc, is most advantageous for the simplification of the construction or cost reduction of the optical pickup apparatus. Here, in order to obtain the common objective optical element for plural kinds of optical discs which use different wavelengths for recording/reproducing information, it is required that the objective optical system is provided with a phase structure having a wavelength dependency for the spherical aberration, which is formed thereon.
European patent application EP-A 1304689 discloses an objective optical system which has the diffractive structure as a phase structure and can be commonly used for the high density optical disc and the conventional DVD and CD, and also discloses an optical pickup apparatus in which this objective optical system is mounted.
Herein, compatibility between different optical discs can be ensured in the optical pickup apparatus written in EP-A 1304689 by designing the diffractive structure of the objective optical system so as to change a diffraction angle according to the wavelength of a light flux which passes through the diffractive structure. However, most diffractive structures for such optical pickup apparatus are generally designed so as to provide an excellent light utilization efficiency when it is used for, for example, the high-density optical disc or DVD. Particularly, when the information recording light for CD enters into the diffractive structure, the diffractive structure sometimes generate unnecessary diffracted light with a diffraction order which is different from the predetermined diffraction order of the diffracted light which is used for information recording and/or reproducing in addition to the diffracted light used for information recording and/or reproducing. When the unnecessary diffracted light enters into a photodetector, there is a fear that some trouble over correct signal reading may be caused.